1. Field of the Invention
The present invention relates to a control method and device for an AC (alternating current) motor, and more particularly, to a control method and device for an AC motor suitable for transforming output power of a converter to AC power by an inverter and driving the AC motor with the AC power.
2. Description of Related Art
There is known a device for controlling an AC motor using a converter and an inverter, for example as disclosed in Japanese Patent Laid-Open No. 61(1986)-109491. Such a conventional device employs a technique of adding a value representing a current in the inverter input side to a current command which is adapted to compensate for a deviation between a DC (direct current) voltage command for the converter output voltage and a detected value of the converter output voltage, setting the summed value as an input current command in the converter output side, and controlling the converter such that the current in the converter output side coincides with the input current command. With this technique, when the load torque of the motor increases abruptly, the current in the inverter input side is also increased. This increase in the current is detected to control the converter so that a current is let to flow into a smoothing capacitor in the converter output side for reducing voltage fluctuations of the smoothing capacitor.
In the prior art, however, no consideration has been paid to the fact that the current in the inverter input side contains a lot of ripple components. If the current in the converter output side contains lower- and higher-order ripple components, the corresponding lower- and higher-order ripple components occur in a current in the converter input side as well. More specifically, when an AC motor is driven using an inverter, a smoothing capacitor in the converter output side is supplied with a working current flown from the smoothing capacitor toward the motor during a working period of the motor and a regenerative current flown from the motor toward the smoothing capacitor during a regenerating period of the motor as positive and negative currents for each switching cycle. As a result, ripple components occur in the inverter input side. These ripple components can be removed completely by providing a filter in the inverter input side which has a time constant longer than the switching cycle. But this lowers the control response speed. For instance, in the case where a high-capacity inverter has a switching frequency as low as 50 Hz (2 ms), it is thought that the filter requires a time constant of about 5 ms. If such filter is built in the inverter input side, the response speed of current control of the converter would be lowered, fluctuations in the DC voltage would be increased, and the motor would be subjected to torque fluctuations.
Furthermore, even when the load torque of the motor is kept constant, a current of a low-order higher frequency six times the primary frequency of the motor occurs in the converter output side. For instance, in the case where the primary frequency is 10 Hz (a 100 ms cycle), there flows an output current of the smoothing capacitor in the same pattern for each period of 16.7 ms (corresponding to a phase angle of 60.degree.). During the region of 60.degree., the DC current value is changed. In addition, the current pattern during the region of 60.degree. varies dependent on a power-factor angle of the motor. Even with the load torque being kept constant, therefore, the capacitor output current produces ripples having a 60.degree. cycle. If the capacitor input current is instantaneously modified to compensate for variations in the capacitor output current, the DC voltage could be suppressed from fluctuating. For the converter in which a source current is controlled to be a sinusoidal current with power-factor of 1, however, the magnitude of the source current is also fluctuated due to some fluctuations in the capacitor input current, resulting in a fear of increasing a distortion factor of the source current.